Laser diodes are typically used to transmit data information over fiber optic networks. To achieve higher speed data rates, a laser diode can be biased with a drive current so it is ‘ON’ and produces at least a minimal optical output. While the diode is biased, the diode can be driven with additional current so that the light output of the diode varies over time between two power output levels. One power output level of the diode can represent a logic low or zero while another power output level of the diode can represent a logic high or one. The optical extinction ratio is the term applied to the relationship in dB between the logic one optical power level and the logic zero optical power level.
A burst mode laser, in contrast to a continuous mode laser, produces output only during selected intervals. It will be appreciated that the burst-mode transmitter is essentially turned off and does not transmit an optical signal until a burst-mode incoming signal is received. Only upon receiving the incoming signal will the burst-mode transmitter operate in comparison to the constant transmission of optical signals at the output of the conventional transmitters. It will be appreciated that the incoming signals can be of various lengths of data, where some signals can be as short as 10 microseconds, for instance, in the case of a DOCSIS burst signal. In a Gigabit PON (GPON), the minimum burst time is 32 ns including the preamble, delimiter and data. The minimum amount of data per burst is 1 byte (6.4 ns)
The laser output power is set by the amount of current passing through the laser. Typically, a bias current is applied to bring the laser up to its threshold and then a modulation current is added to amplitude modulate the laser with a baseband digital signal. The relationship between modulation current and laser output power is commonly referred to as “slope efficiency”. As temperature increases, the laser threshold increases and the slope efficiency decreases. As temperature decreases, the laser threshold decreases and slope efficiency increases. As the laser ages, laser threshold increases and slope efficiency decreases. Laser aging is very similar to operating at a higher temperature.
To compensate for temperature fluctuations and aging, many laser driver control circuits employ an analog control loop to maintain a constant average output power from the laser. A power monitor photodiode senses the output power of the laser for feedback to the driver control circuit. In particular, the power monitor photodiode typically receives a portion of the output power from the back facet of the laser and generates a current that is proportional to the output power from the front facet of the laser. The front facet of the laser is aligned with the fiber core to create a signal output path.
The laser driver control circuit may include an analog loop that compares the photodiode current to a reference current value. Based on the comparison, the driver control circuit adjusts the bias current to reduce the error between the photodiode current and the reference current. In some circuits, as an alternative, the photodiode current is applied to a resistor to produce a monitor voltage indicating the output power of the laser. The driver control circuit then compares the monitor voltage to a reference voltage and controls the laser drive current to reduce error.
The analog control loop that is employed may be an open loop or closed loop circuit. In an open loop circuit, temperature indexed look up tables are used for both the bias and modulation currents. One obstacle to implementing this method is the generation of the look up table itself, which is a time consuming process. In addition, the problem of laser aging is generally handled by setting the laser power to the maximum allowed by its specification and then letting the laser fail over time. Unfortunately, this has the added effect of prematurely aging the laser.
Closed loop control circuits generally handle temperature fluctuations and laser aging better than open loop circuits. Closed loop control circuits may employ a single closed loop or a dual closed loop. In a single loop control circuit, the average power level is measured during each burst using the photodiode as a monitor. The bias current is then adjusted in order to keep a constant current out of the photodiode. The modulation current is typically adjusted using a temperature indexed look up table. Unfortunately, this technique is not an effective way to maintain a high extinction ratio without risking severe eye diagram distortion.
In a dual closed loop control circuit, the bias current is adjusted until the desired logic level zero is achieved and the modulation current is adjusted to keep the average transmit power during each burst at a desired level. While in principle a dual closed loop control circuit provides the best response, it can be difficult to implement, particularly when very high extinction ratios are employed. This is because high extinction ratios require very low logic zero power levels, in some cases less than 10 microwatts, which can be very difficult to measure. To ensure an accurate measurement of the zero power level, it typically will be necessary to measure an extended string of zeros.